Thin-film resistor adjustment

ABSTRACT

An apparatus for adjusting the resistance values and temperature coefficients of resistance of thin-film resistors within very close tolerances. The adjustment is achieved through heating of the thin-film resistors to a very high temperature for very short periods of time.

United States Patent 72] Inventors John W. Ireland;

James C. Holloway, both of Canoga Park, Calif. [21] Appl. No. 852,132[22] Filed May 26, 1969 Division of Ser. No. 410,439, Nov. 12, 1964,Pat. No. 3,457,636 [45] Patented Sept. 7, I971 [73] Assignee TheBunker-Homo Corporation Stamford, Conn.

. [s4 1 THIN-FILM RESISTOR ADJUSTMENT l0 Claim, 11 Drawing Pb.

I 324/62 [51] lot. (I 1105b 1/02 [50] Field-of Search 219/499; 323/75;374/62 B, 63

m 35 b so Primary Examiner-Bernard A. Gilheany Assistant Examiner-F. E.Bell Attorney-Frederick M. Arbuclde ABSTRACT: An apparatus for adjustingthe resistance values and temperature coefficients of resistance ofthin-film resistols within very close tolerances. The adjustment isachieved through heating of the thin-film resistors to a very hightemperature for very short periods of time.

Tl-lIN-FILMRESISTOR ADJUSTMENT This application is'a division ofcopending application Ser. No. 410,439, filed Nov. 12, 1964, and now US.Pat. No. 3,457,636.

This-invention relates to thin-film resistors, and, more particularly,to methods and means for adjusting the resistance valuesand temperaturecoefficients of resistance of such re- 'sistors.

Thin-film resistors per se are known in the art. Such resister'sgenerally comprise a very thin film of metal coated on a sglassorceramic substrate. Various metals have been used for the resistive film,amongthembeing chromium, Chromel-C (a member of the Niero'me familycomprising chromium, nickel and iron) and Cermet (comprising chromiumand silicon oxide). S'uch resistors,as'manufactured, do not normallyprovide better than a-iS percent tolerance. This,'of course, has imposedsevere Iimitatiorison circuits requiring more precise resistertolerances, andhas required that the resistors be physically alteredtobring them to'closer tolerances. Such alteration has generally'beenaccomplished by removing-material from the (resistor itself by scribing,etching, or cutting, or by the use of shorting bars, thatis, conductorsdeposited across the resistor at points between the end electrodes;Scribing and cutting tend to promote unpredictable aging effects andhence loss of tolerance, and etching is not easily controllable.Shortingbar's cannot-provide tolerances that are sufficiently close formodern military circuits, say il percent. Furthermore, such physicalalterationdoesnot afi'ect the temperature'coefl" ficient of resistanceof the resistor in any way.

Recent techniques have led'to the development of integral thin-filmnetworks; in which the substrate carries both resistors and conductors.Heretofore, no technique has been available for altering thevalues'ofindividual'resistors in sucha multiresistor network except-thosetechniques previously mentioned which involve physically altering eachresiston'The present invention eliminates the disadvantagesofsuchtechniquesand provides methods and means for adjusting the resistancevalues of thin film'resisto'rs quickly andwith a high degree ofaccuracy. Furthermore, the invention" providesa technique whereby thetemperature coefficient of resistance r of a thin-film resistor maybea'djusted independently of the re-* sistancevalue; I

The invention is based-on the discovery that the resistance of athin-film resistor'may 'be adjusted to close tolerance by' heating theresistor to a high temperature for a very short periodof time. While theresistance of the resistor is being adjuste'd, the temperaturecoefiicient'of resistance (TCR) of the resistor may also be adjusted tovery close tolerance. Furthermore; the TCR of the resistor may beadjusted without per-' manently changing the resistance value of theresistor, or, with a change in resistance value of a predeterminedamount.

Each resistor of a multiresistor thin-film networkmay-beim" dividuallyadjusted utilizing the teachings of the invention.

decre'aseinit'ially and thenwill start to rise with the passage of time.In both cases, the TCR of the resistor increases. If a coated-resistoris heated according to 'the invention, its re sistance value willdecrease whileits TCR will increaseiThisis believed to be due toannealing of the resistor material.

According to another feature of the invention, an uncoated oxidiz'ableresistor may be heated in inert and oxidizing atmosphere, one after theother, to adjust the TRC of the resisto'r to a desired value. Duringthat process, the resistance value of the resistor may be adjusted to apredetermined desired value, or the processmaybe so carried out as toproduce no pennanent change in the resistance value of the resistor.

In the event that individual-resistors are heated by passing electricalcurrent throughthem, mea'ns'are provided-for auto matically monitoringthe resistsnce value as theresistor is being heated, and for terminatingthe heating when'a desired value has been attained; I

The invention, together witht urther advantages and features thereof,will be better understood' from the following description of severalembodiments; taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a plan view of asimplified-multiresistor-thin-film network;

FIG. 2is a graph showing typical resistance adjustments as 'a function'of adjustment time:

FIG. 3 isa graphsho'wing' resistanceadjustmentas a function ofadjustment timeof a specific thin-film resistance.

material;

no; 4 is a graph showing the changein TCR as a function of resistanceadjustmentof two specific thin-fiIm-r'esis'tance materials;

FIGS. Sand 6 'are'graphs-useful inunderstanding the sim'ul taneousadjustment of resistance and TRC,

FIG. 7 is a circuitdi'agram of meansfo'rindividually adjust-- ing'a-pluralityof resistors bypassing electric c-urren'tthroughw them;

FIG. 8-is a perspective viewof a multiprobe headuseful in a.

adjusting'resistances according to otherembodiments ot the invention;

FIG. 1' illustrates-a rn'ultiresistor thin film. network '10,

shown in'much simplified-form. In actual practice, such networks areusually quite complex-and-comprise many resistors and conductors. Asshown, the" network locomp'rises-a sub-' strate ll ,-which carriesaplurality of resistors*mh lldanda plurality of conductors 130-132. Thesubstrate ll" generally consists of glass or glazed ceramic, with -glassbeing used -in I most instances because of its lower cost'and'smoothersurface.

The resisto'rs 12' may consist of a relatively high-resistance materialsuchas chromiuimCh'romel-C, or Cermet, theiatter two materials alsocontaining chromium. Thep'arti'cular type of resistive material used isdependent upon the desired characteristics of thefinal resistor, andthepr'esent invention is m ne way limited to the use of any particularresistive material. Characteristics of three typical resistive'materialsare shown in :Table I. Particular values within tlievarious ranges shownin the'table'may be obtaineduby control of the process by means 'ofwhich the material-is deposited :on the substrate,as is wellkn'own inthe art.

TABLE I Material Resistance TCR (Ohms/Square) (parts per million! C.)

Chr'omeLC 25 R5500 -25 to +25 Chrorniui'n 25 to 500 a -250 to 0 Cermet$O0 to 50 The conductors 13 generally consistof a very low resistancematerial such as gold or gold co'pp'er. Here again, however,-

the invention is not limited to the use of any particular materials forthe conductors.

The resistors 12 and the conductors 13 may be deposited on the substrate11 by any one of various known means. For'ex ample, they may be vacuumdeposited by evaporation or by sputtering, or they may be plated on thesubstrate. In addition,

the resistors may be left uncoated or they may be coated with aprotective material such as silicon monoxide, depending upon the use towhich the network is to be put and the atmosphere in which it will beused. The problem that the present invention solves is that ofindividually adjusting the resistors l2a-12d quickly and to very closetolerances. As previously mentioned, adjusting the resistors hasheretofore been done by physically altering their geometry such as byabrading or etching the individual resistors. Such processes are timeconsuming and do not provide resistance value tolerances sufficientlylow for use in many modern day circuits.

A solution to the foregoing problem is provided by the presentinvention, which is based upon the discovery that the resistance valuesof the various resistors of the network can be individually adjusted ina matter of seconds by heating the individual resistors, one at a time,to a high temperature. The heating process may be accomplished either bypassing an electrical current through each resistor, by bringing a smallheated filament over the resistor, or by applying hot gas directly onthe resistor. The specific methods and means used in heating theresistors will be described in detail hereinafter. First, however, thetemperature to which each resistor is heated, the period of time forwhich it is heated, and the result on the characteristics of theresistor will be considered.

It is well known that the resistance value of a thin-film resistor isrelatively unstable without further treatment after deposition. Thiscondition, of course, cannot be tolerated in precision circuitry.Therefore, the resistor is aged at an elevated temperature to stabilizeits resistance value so that it will undergo very little, if any, changeduring the life of the resistor. In the present case stabilizing isaccomplished by aging the resistor for 18 or 20 hours at a temperatureof 250 C. It may conveniently be accomplished by placing the substratecontaining the entire network in an oven for the required length oftime. During the aging and stabilizing process, the resistance and theTCR of an uncoated resistor increase; during that process the resistanceof a silicon-monoxide coated resistor decreases while its TCR increases.Thus, caution must be taken to insure that the initial resistance valuesare not too high or too low to permit proper stabilizing without theresistance and TCR values exceeding the desired values.

It has been discovered that, after a thin-film resistor has had itsresistance value stabilized, its resistance value may be adjusted byfurther heating the resistor at a more elevated temperature for a shortperiod of time. Although the temperature to which the resistor is heatedand the length of time for which it is heated vary inversely with eachother (although not proportionally) for a given change in resistance, itis preferred that the resistor be heated at a relatively hightemperature for a relatively short period of time. For example, if theresistor is heated to a temperature higher than 300 C. a period of lessthan 30 seconds is required to change its resistance by the maximumextent possible. FIG. 2 represents curves showing the amount ofresistance adjustment possible utilizing the method of the invention. Acurve 16 shows the increase in resistance of an uncoated resistor as itis heated for various periods of time, and a curve 17 shows the decreasein resistance of a coated resistor as it is heated for various periodsof time. As shown, the resistance value of a resistor may be adjusted bymore than 100 percent. However, in practice, resistors are not adjustedby more than about 25 percent of their initial value. In other words,the adjustment takes place on the relatively steep portion of the curvesl6 and 17 in a time of seconds or less. FIG. 3 shows an actual curve ofresistance versus adjustment time for Chromel-C which was heated bypassing current through the resistor.-

As previously noted, whether the resistance value of a resistor isadjusted upwardly or downwardly, its TCR always increases. This is shownby FIG. 4, which represents the change in TCR as a function of thepercent resistance adjustment for a coated resistor made of Chromel-Cand for an uncoated chromium resistor. The increase in TCR for Chromel-Cis approximately 3 parts per million (ppm) per degree centigrade foreach 1 percent change in resistance, as shown by a curve 18. The changein TCR for chromium is much greater, as shown by a curve 19, andapproximates 10 parts per million per degree centigrade for each 1percent change in resistance.

One of the outstanding features of the invention is that the TCR of aresistor may be adjusted independently of adjustment of the resistancevalue of the resistor. This is shown graphically by the curves presentedin FIGS. 5 and 6. These figures are based on data derived from heatinguncoated chromium resistors in oxidizing and in inert atmospheres.Looking first at FIG. 5, a curve 20 represents the change in TCR versusthe percent resistance adjustment of the chromium resistor when it isheated to a temperature in the range of 425 C. to 450 C. in an oxidizingatmosphere. Similarly, a curve 21 represents the change in TCR versuspercent resistance adjustment when the resistor is heated in an inertatmosphere. It is apparent from the curve 21 that the resistance valueof the resistor first decreases and then increases as the TCR increaseswith continued heating of the resistor. By combining the effects ofheating in inert and oxidizing atmospheres, various changes in TCR andresistance can be obtained simultaneously. For example, if it is desiredto increase the resistance value of a resistor by approximately 9percent and increase its TCR by approximately 225 parts per million, theresistor could first be heated in an inert atmosphere until itscharacteristics correspond to those indicated at point 21a and thencould be heated in an oxidizing atmosphere until the desiredcharacteristics are obtained at point 20a on curve 20. Similarly, if itis desired to adjust the resistance upwardly by approximately 10 percentand the TCR upwardly by approximately 280 parts per million, theresistor could be heated in an inert atmosphere until itscharacteristics are as indicated at point 21b, and then heated in anoxidizing atmosphere until its characteristics reached their desiredvalues at point 20b on curve 20". Of course, the resistor could beentirely adjusted in an inert atmosphere or in an oxidizing atmosphereif the desired characteristics could be obtained in that manner.

FIG. 6 represents a situation which is the reverse of that shown in FIG.5, in that a resistor is first adjusted in an oxidizing atmosphere sothat its characteristics follow along the curve 20 and is then furtheradjusted in an inert atmosphere. For example, if it is desired toincrease the TCR of a resistor by approximately parts per million andincrease its resistance by approximately 5 percent, the resistor mightfirst be heated in an oxidizing atmosphere until its characteristicscorrespond to those at point 200 on the curve 20 and then further heatedin an inert atmosphere until its characteristics are as represented bythe point 21c on curve 21". If it is desired to increase the TCR andresistance of the resistor by a greater amount, it might be treated inan oxidizing atmosphere until its characteristics correspond to thepoint 20d, after which it would be treated in an inert atmosphere untilits characteristics corresponded to the desired point on curve 21 It isunderstood, of course, that the particular curves presented in FIGS. 5and 6 are merely representative ones and that many other changes in TCRand resistance value can be obtained by sequentially heating a resistorin inert and oxidizing atmosphere, one after the other.

As was previously mentioned, heating of a resistor to the desiredtemperature range of 425 C. to 450 C. may be accomplished by passingelectrical current through the resistor, by bringing a hot probeadjacent to the resistor, or by directing hot gases against theresistor. FIG. 7 is a circuit diagram of means for individuallyadjusting a plurality of resistors by passing electrical current throughthem. Although direct current can be used for heating a thin-filmresistor in accordance with the invention, it has been found thatalternating current is preferable. Therefore, power is supplied to thecircuitry shown in FIG. 7 by an alternating current source 30, which maybe a conventional 60-cycle, l l0-volt supply.

Essentially, the circuitry shown in FIG. 7 comprises a bridgearrangement, indicated generally by the numeral 31, and means forselectively energizing and deenergizing the bridge arrangement, suchmeans being indicated generally by the numeral 32.

Consider first the means 32 for selectively energizing and deenergizingthe bridge arrangement 31. The means 32 comprise an autotransformer 33connected in series with a milliammeter 34 across the alternatingcurrent source 30 through a normally closed contact 35a of a relay 35, anormally open contact 36a of a relay 36, and a line switch 38. Relay 35has a coil 35b which is energized from the bridge arrangement 31 in amanner to be hereinafter described, and the relay 36 has a coil 36bwhich is connected across the alternating current source 30 through amomentary contact switch 37 and through the line switch 38. Alsoconnected across the alternating current source 30 is an autotransformer39, between whose movable arm and one end is connected a resistanceheater shown as a resistor 40, the purpose of which will be laterdescribed. A primary winding 41a of a transformer 41 is connectedbetween the movable arm and one end of the autotransformer 33 and powerfor the bridge arrangement 31 is derived from a secondary winding 41b ofthe transformer 41.

The bridge arrangement 31 comprises a Wheatstone bridge having fourarms, two adjacent arms of which comprise variable resistors 42 and 43.The other two arms of the bridge arrangement 31 are adapted to haveresistors to be tested and standard wire-wound resistors, respectively,sequentially switched into them. As shown, a plurality of resistors 44a-4411, whose resistance values are to be adjusted, are respectivelyconnected between pairs of contacts of a first section .450 oftwo-section stepping switch. A second section 45b of the steppingswitch, which is mechanically connected to the first section 45a, hasconnectedbetween its plurality of pairs of contacts a plurality ofstandard resistors 46a--46n. Each section of the switch is provided witha pair of poles which may be sequentially moved between the variouspairs of contacts. The resistors 44 to be adjusted and the standardresistors 46 are so arranged with respect to each other that when aswitch is positioned to put, for example, the resistor 44n into theWheatstone bridge, the standard resistor 46" is placed in the adjacentarm of the bridge. The standard resistor, of course, represents theresistance value to which the resistor 44 is to be adjusted.

The bridge 31 is energized from the transformer 41 by connecting pointsbetween the variable resistors 42 and 43 and between the resistors 44and 46 to opposite ends of the secondary winding 41b of the transformer.Detector means 47, such as a null detector or a phase detector, isconnected between the other two juncture points of the bridge. Thedetector means 47 functions to provide an output signal when the bridge31 is balanced. The output signal of the detector means 47 is connectedto energize the coil 35b of the relay 35 in the energizing section 32.

To operate the adjusting means shown in FIG. 7, the line switch 38 isfirst closed and then the momentary contact switch 37 is closed. Thisenergizes the coil 36b of the relay 36, which in turn closes thenormally open contact 36a, which energizes the autotransformer 33 andthe transformer 41. When the contact 36a is closed, power is supplied tothe relay coil 36b until the contact of 35a of relay 35 opens. It isassumed, of course, that the resistors 44 to be adjusted and thestandard resistors 46 have been properly connected into the steppingswitch 45 and the switch set to a desired position beforethe circuit isenergized.

Current from the secondary winding 41b of the transformer 41 flowsthrough the bridge arrangement 31, including a resistor to be adjusted,and a standard resistor. The current flowing through the resistor to beadjusted heats the resistor to be adjusted so that its resistance valueand TCR are varied in the manner heretofore described. The amount ofcurrent flowing through the resistor to be adjusted is controlled by thesetting of the autotransformer 33. An indication of that current isprovided by the milliammeter 34 in series with the autotransformer 33.

When the resistance value of the resistor 44 being adjusted has reachedthat of the standard resistor 46 to which it is being compared, thedetector means 47 provides an output which energizes the coil 35b of therelay 35. This, in turn, opens the normally closed contact 35a of therelay and deenergizes the circuit. The stepping switch 45 may then beadvanced to the next position and another resistor adjusted in themanner previously described. The process is thus continued until all ofthe resistors 44 have been adjusted. The standard resistors 46 are muchlarger physically than the resistors 44. Thus they easily dissipate theheat caused by the current flowing through them and their values remainunchanged.

It is pointed out that resistor adjustment using circuitry such as shownin FIG. 7 may be carried out in either an inert or an oxidizingatmosphere in the manner heretofore described.

One of the features of the present invention is that the circuitry shownin FIG. 7 cube used to individually adjust each resistor in amultiresistor thin-film network of the type shown in FIG. 1. Electricalcontact may be made with the resistors on a substrate by using amultiple-probe fixture such as is shown in FIG. 8, which is designed forthe particular resistor configuration on the substrate. Themultiple-probe fixture may comprise a plate of insulating material 50having a plurality of spring-loaded electrical probes Sla-Sle extendingthrough holes in the plate. The probes 51 are so arranged that when thefixture is placed in contact with a substrate bearing a multiresistornetwork, each of the probes contacts one of the conductors. The probesare so arranged in the fixture shown in FIG. 8 that if it is used totest the network shown in FIG. 1, the probes 510 through 5lerespectively contact the conductors 13a through 132 on the substrate.The probes are electrically connected to the contacts of the switchsection 45a of the switch 45 shown in FIG. 7. It is particularly pointedout that a fixture must be specially designed for each substrate networkconfiguration. The plate 50 also has a plurality of apertures 50atherein which are used for registration purposes as will be laterdescribed.

FIG. 9 illustrates an adjustment assembly utilizing the multiprobefixture 50 previously described with reference to FIG. 8. The assemblycomprises a base 55 made of a suitable material on which is mounted asubstrate holder and heater 56. The substrate holder and heater 56 mayconsist of a stainless steel plate which is heated by an electricallyinsulated nicrome ribbon. The heater ribbon is not shown but conductorsthereto are indicated by the numeral 57. The conductors 57 are connectedto the autotransforrner 39 shown in FIG. 7, wherein the heater ribbon isrepresented by the resistor 40.

It has been found desirable to heat the substrate during the resistoradjustment process to reduce thermal stress in the substrate and also toaid in the adjustment itself. Also, by heating the substrate, a lowercurrent through the resistor under adjustment may be used to achieve agiven rate of adjustment. Typical temperatures for the substrate holderand'heater 56 for various resistor material configurations are shown inTable II.

The substrate holder and heater 56 is provided with a plurality of pins56a extending from its upper surface which define the position of thesubstrate 11 and between which the substrate fits. The substrate holderand heater 50 is also provided with a second plurality of pins 56b,which fit into the openings 50a in the multiple-probe fixture 50 toinsure proper registration of the probes 51b with the conductors on thesubstrate 11. After the substrate is properly positioned on the holderand heater 56, the multiple-probe fixture 50 is lowered into position sothat the electrical probes 51b engage the conductors of themultiresistor network carried by the substrate. The fixture 50 may beheld in position, while the adjustment of the resistor proceeds, byconventional means such as springs, weights or clamps.

Table III presents data relating to six typical resistors whoseresistance values were adjusted by passing electrical current throughthe resistors. All of the resistors were adjusted in an oxidizingatmosphere so that the resistance values of the uncoated resistorsincreased. The data relating to the power in watts per square inchdissipated by the resistor being adjusted relates to the powerdissipation at the beginning of the adjustment period. Of course,assuming that the voltage across the resistor is maintained constant, asthe resistance value increases, the power dissipated decreases.Conversely, if the resistance value is adjusted downwardly, as in thecase of a coated resistor, the power dissipation increases during theadjustment process.

As was previously mentioned, each resistor of a multiresistor thin-filmnetwork may be heated by means other than by passing electrical currentthrough it. FIG. 10 illustrates, in diagrammatic form, an arrangementwhereby a resistor 60 having end conductors 61 is adjusted by means ofradiant heat. The resistor 60 is carried on a substrate 62 which ismounted on a heated holder 63. The resistor is heated for adjustment bybringing a probe 64 having a heating element 65 adjacent the surface ofthe resistor. The heating element 65 may conveniently be a nicrome wirewhich is heated to a temperature of 900-l ,000 C. by passing electricalcurrent through it. The heating element is brought to within anapproximately 0.01 inches of the resistor surface to efi'ect theadjustment. During the adjustment process, the resistance value of theresistor 60 may be monitored by an ohmmeter (not shown) connectedbetween the end conductors 61 by means of leads 66. Of course, when theresistance value has reached the desired value the probe and heatingelement are removed from the vicinity of the resistor. Again, as withthe electrical adjustment method previously described, the holder 63 onwhich the substrate is mounted is heated to reduce the stress in thesubstrate and aid in the adjustment process.

Table IV presents data for four typical uncoated chromium resistorswhich have been adjusted by means of radiant heat as shown in FIG. 10.The data relating to TCR is in parts per million per degree centigradechange in temperature.

TABLE IV Before Adjustment After Adjustment FIG. 11 illustratesdiagrammatically still another method for adjusting the resistance valueof a thin-film resistor. As shown,

a thin-film resistor having end conductors 71 is carried by substrate72. During the adjustment procedure, the substrate 72 is mounted on aheated holder 73 as in the methods previously discussed. This embodimentdiffers from those previously described, however, in that hot gases aredirected against the resistor to heat it to the proper temperature foradjustment. A cylinder 74 of glass or other appropriate material isplaced about the resistor 70 with sufi'rcient space being left betweenthe bottom of the cylinder and the top of the substrate 72 for gas toescape freely. Hot gas is directed against the surface of the resistor70 through nozzles 75 and 76. One

of the nozzles 75 and 76 may be connected to a source of hot inert gas,such as hydrogen, helium, argon or neon, and the other nozzle may beconnected to a source of hot oxidizing gas such as air. Thus, theresistance value of the resistor 70 mat be adjusted in either an inertor an. oxidizing atmosphere or in both, one after the other, aspreviously described. The resistance value may be monitored during theadjustment process by means of an ohmmeter (not shown) connected betweenleads 77 which are electrically connected to the end conductors 71.

It is now apparent that the invention provides methods and means forquickly and accurately adjusting the values of resistors in an integralthin-film network. Each resistor in the network has its resistance valueadjusted independently of all other resistors in the network. Resistorsso adjusted have been found to maintain their resistance values tobetter than 10.1 percent after thousands of hours of use. Furthermore,the TCR of a resistor may be adjusted without permanently altering theresistance value. It is pointed out that various parameters of theadjustment process may be determined empirically to fit differentsituations without departing from the true spirit and scope of theinvention.

What is claimed is:

1. Apparatus for automatically adjusting the impedance value of aheat-adjustable impedance, said apparatus comprising in combination:

impedance bridge means to which said heat-adjustable impedance iselectrically connected so as to permit Measurement of the impedancethereof power source means for energizing said bridge means and forheating said heat-adjustable impedance so as to cause its impedancevalue to vary in a predetermined manner,

detector means coupled to said bridge means for detecting when saidheat-adjustable impedance reaches a desired predetermined value and forproviding an output indication in response thereto, and

means coupled to said power source means and to said detector means forterminating the heating of said heat-adjustable impedance in response tothe occurrence of said output indication.

2. The invention in accordance with claim 1,

wherein said heat-adjustable impedance is a thin-film resistor supportedon a substrate, and

wherein said apparatus includes electrical probes for electricallyconnecting said resistor to said bridge means.

3. The invention in accordance with claim 2, wherein said apparatusincludes means for heating said substrate during adjustment of saidresistor.

4. The invention in accordance with claim 2, wherein the heating of saidresistor is obtained as a result of the current flowing therein whensaid bridge means is energized by said power source means.

5. The invention in accordance with claim 4, wherein the heating of saidresistor is terminated by interrupting the flow of current therein.

6. The invention in accordance with claim 5, wherein said power sourcemeans supplies an alternating current to said bridge means.

7. The invention in accordance with claim 5,

wherein said apparatus includes means for selectively energizing anddeenergizing said bridge means, and

wherein the deenergizing of said bridge means is caused to sistors tosaid bridge means for adjustment thereof.

9. The invention in accordance with claim 8, wherein the heating of eachresistor when connected to said bridge means is obtained as a result ofthe current flowing therein when said bridge means is energized by saidpower source means.

10. The invention in accordance with claim 9, wherein said apparatusalso includes means for heating said substrate during adjustment oi saidsistors.

1. ApparaTus for automatically adjusting the impedance value of a heat-adjustable impedance, said apparatus comprising in combination: impedance bridge means to which said heat-adjustable impedance is electrically connected so as to permit Measurement of the impedance thereof power source means for energizing said bridge means and for heating said heat-adjustable impedance so as to cause its impedance value to vary in a predetermined manner, detector means coupled to said bridge means for detecting when said heat-adjustable impedance reaches a desired predetermined value and for providing an output indication in response thereto, and means coupled to said power source means and to said detector means for terminating the heating of said heat-adjustable impedance in response to the occurrence of said output indication.
 2. The invention in accordance with claim 1, wherein said heat-adjustable impedance is a thin-film resistor supported on a substrate, and wherein said apparatus includes electrical probes for electrically connecting said resistor to said bridge means.
 3. The invention in accordance with claim 2, wherein said apparatus includes means for heating said substrate during adjustment of said resistor.
 4. The invention in accordance with claim 2, wherein the heating of said resistor is obtained as a result of the current flowing therein when said bridge means is energized by said power source means.
 5. The invention in accordance with claim 4, wherein the heating of said resistor is terminated by interrupting the flow of current therein.
 6. The invention in accordance with claim 5, wherein said power source means supplies an alternating current to said bridge means.
 7. The invention in accordance with claim 5, wherein said apparatus includes means for selectively energizing and deenergizing said bridge means, and wherein the deenergizing of said bridge means is caused to occur in response to the occurrence of said output indication.
 8. The invention in accordance with claim 2, wherein said apparatus is constructed and arranged for automatically adjusting the resistance value of each of a plurality of heat-adjustable thin-film resistors supported on a common substrate, and wherein said apparatus includes automatically operating switch means for successively connecting each of said resistors to said bridge means for adjustment thereof.
 9. The invention in accordance with claim 8, wherein the heating of each resistor when connected to said bridge means is obtained as a result of the current flowing therein when said bridge means is energized by said power source means.
 10. The invention in accordance with claim 9, wherein said apparatus also includes means for heating said substrate during adjustment of said resistors. 